Method for preparing multilayer of nanocrystals, and organic-inorganic hybrid electroluminescence device comprising multilayer of nanocrystals prepared by the method

ABSTRACT

A method for preparing a multilayer of nanocrystals. The method includes the steps of (i) coating nanocrystals surface-coordinated by a photosensitive compound, or a mixed solution of a photosensitive compound and nanocrystals surface-coordinated by a material miscible with the photosensitive compound, on a substrate, drying the coated substrate, and exposing the dried substrate to UV light to form a first monolayer of nanocrystals, and (ii) repeating the procedure of step (i) to form one or more monolayers of nanocrystals on the first monolayer of nanocrystals. Further, an organic-inorganic hybrid electroluminescence device using a multilayer of nanocrystals prepared by the method as a luminescent layer.

This is a divisional application of U.S. patent application Ser. No.12/432,180 filed on Apr. 29, 2009, which is a divisional application ofU.S. application Ser. No. 11/002,461 filed on Dec. 3, 2004, and claimspriority under 35 U.S.C. 119(a) to Korean Patent Application No.2004-38391 filed on May 28, 2004, which is herein expressly incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing a multilayer ofnanocrystals, and an organic-inorganic hybrid electroluminescence devicecomprising a multilayer of nanocrystals prepared by the method. Moreparticularly, the present invention relates to a method for preparing amultilayer of nanocrystals by (i) coating nanocrystalssurface-coordinated by a photosensitive compound, or a mixed solution ofa photosensitive compound and nanocrystals surface-coordinated by amaterial miscible with the photosensitive compound, on a substrate,drying the coated substrate, and exposing the dried substrate to UVlight to form a first monolayer of nanocrystals, and (ii) repeating theprocedure of (i) to form one or more monolayers of nanocrystals on thefirst monolayer of nanocrystals; and an organic-inorganic hybridelectroluminescence device using the multilayer of nanocrystals preparedby the method, as a luminescent layer. The use of the multilayer ofnanocrystals as a luminescent layer can enhance luminescent efficiencyand luminescence intensity of the electroluminescence device, and cancontrol electrical properties of the electroluminescence device.

2. Description of the Related Art

A nanocrystal is defined to be a material having a crystal size at thenanometer-scale level, and consists of a few hundred to a few thousandatoms. Since the nanocrystal has a large surface area per unit volume,most of the atoms constituting the nanocrystal are present at thesurface of the nanocrystal. Based on this structure, the nanocrystalexhibits quantum confinement effects, and shows electrical, magnetic,optical, chemical and mechanical properties different from thoseinherent to the constituent atoms of the nanocrystal. That is, thecontrol over the physical size of the nanocrystal enables the control ofvarious properties.

Vapor deposition processes, such as metal organic chemical vapordeposition (MOCVD) and molecular beam epitaxy (MBE), have beenconventionally used to prepare nanocrystals. On the other hand, a wetchemistry technique wherein a precursor material is added to an organicsolvent to grow nanocrystals to a desired size has made remarkableprogress in the past decade. According to the wet chemistry technique,as the crystals are grown, the organic solvent is naturally coordinatedto the surface of the quantum dot crystals and acts as a dispersant.Accordingly, the organic solvent allows the crystals to grow to thenanometer-scale level. The wet chemistry technique has an advantage inthat nanocrystals can be uniformly prepared in size and shape in arelatively simple manner at low cost, compared to conventional vapordeposition processes, e.g., MOCVD and MBE. However, since nanocrystalsprepared by the wet chemistry technique are commonly dispersed in anorganic solvent, techniques for forming a thin film of the nanocrystalsare required in order to apply the nanocrystals to electronic devices.

Conventionally, a self-assembly process has been mainly used to form athin film of nanocrystals prepared by the wet chemistry technique. Forexample, U.S. Pat. No. 5,751,018 discloses a method for attachingsemiconductor nanocrystals to solid inorganic surfaces, usingself-assembled bifunctional organic monolayers as bridge compounds.Further, Korean Patent Application No. 2002-85262 discloses a method forpreparing a multilayer of nanocrystals by bonding a dithiol group to thesurface of nanocrystals to form disulfide bonds between thenanocrystals. Further, a method is disclosed for preparing a multilayerof nanocrystals by charge-charge interaction. According to this method,nanocrystals surface-substituted with a charged compound are bound to anoppositely charged substrate to form a thin film of the nanocrystals, anorganic compound oppositely charged to the nanocrystals is bound on thethin film, and then the above procedure is repeated.

However, these methods for forming a thin film of nanocrystals byself-assembly have a problem that they involve an additional step oftreating the surface of nanocrystals and a substrate with a compoundcontaining a specific functional group, rendering the overall proceduremore complex.

Meanwhile, electroluminescence devices using nanocrystals as aluminescent layer are described in U.S. Pat. Nos. 5,537,000, 6,608,439and 6,049,090, and PCT publication WO 03/084292.

U.S. Pat. No. 5,537,000 describes an electroluminescence device withoutan organic electron transport layer in which a multilayer ofnanocrystals acts as both a luminescent layer and an electron transportlayer, and the wavelengths of emitted light are dependent on a voltageapplied to the device. However, the patent publication simply describesthat the multilayer of nanocrystals can be prepared by the use of aself-assembly process mentioned in U.S. Pat. No. 5,751,018, but fails tospecifically describe a process for forming a monolayer of nanocrystalsor preparing a multilayer of nanocrystals using the monolayer.

PCT publication WO 03/084292 describes an organic-inorganic luminescencedevice which comprises a matrix containing a large number ofnanocrystals and disposed between two electrodes. This patentpublication also suggests a method for enhancing electroluminescentperformance of the device by spin-coating a mixture of nanocrystals anda low-molecular weight hole transporting material (e.g.,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)4,4′-diamine(TPD)) on an electrode, and introducing an organic electron transportlayer and electron/hole blocking layers into the device, therebyallowing the organic layer to transport electrons and holes and allowingthe nanocrystals to emit light.

Further, U.S. Pat. No. 6,608,439 discloses an integrated organic lightemitting diode color display device in which nanocrystals used as acolor-conversion layer absorb monochrome and short-wavelength lightemitted from an organic layer, and then emit photoluminescence (PL) at adifferent wavelength. However, the luminescence device is not driven byelectroluminescence.

Further, U.S. Pat. No. 6,049,090 describes a device using a mixed layerof nanocrystals-matrix as a luminescent layer disposed between twoelectrodes wherein the band gap energy and the conduction band energylevel of the matrix are more than those of the nanocrystals. The patentpublication also explains that since electrons and holes are trapped bythe matrix, the luminescent efficiency of the device can be enhanced.

As described above, there are few reports on the preparation of amultilayer of nanocrystals used as a luminescent layer in order toenhance the luminescent efficiency of electroluminescence devices,except for the self-assembly process involving complicated steps.

OBJECTS AND SUMMARY

Thus, the present inventors have earnestly and intensively conductedresearch to solve the problems of conventional methods for preparing amultilayer of nanocrystals in terms of complicated procedures. As aresult, the present inventors have found that when nanocrystalssurface-coordinated by a photosensitive compound are coated on asubstrate and exposed to UV light, a crosslinking reaction takes placebetween photosensitive functional groups of the photosensitive compoundto form a network structure, resulting in the formation of a monolayerof nanocrystals insoluble in solvents, and further the preparation of amultilayer of nanocrystals in a relatively simple manner when comparedto prior art methods. The present invention is based on this finding.

Therefore, it is an object of the present invention to provide a methodfor preparing a multilayer of nanocrystals in a simple manner, withoutthe need for a complicated process, such as self-assembly ofnanocrystals.

It is another object of the present invention to provide a method forpreparing a multilayer of nanocrystals-polymer by utilizing the methodfor preparing a multilayer of nanocrystals.

It is yet another object of the present invention to provide anelectroluminescence device with enhanced luminescent efficiency using amultilayer of nanocrystals or a multilayer of nanocrystals-polymerprepared by any one methods of the present invention, as a luminescentlayer.

In accordance with one aspect of the present invention, the aboveobjects can be accomplished by a method for preparing a multilayer ofnanocrystals comprising the steps of: (i) coating nanocrystalssurface-coordinated by a photosensitive compound, or a mixed solution ofa photosensitive compound and nanocrystals surface-coordinated by amaterial miscible with the photosensitive compound, on a substrate,drying the coated substrate, and exposing the dried substrate to UVlight to form a first monolayer of nanocrystals; and (ii) repeating theprocedure of step (i) to form one or more monolayers of nanocrystals onthe first monolayer of nanocrystals.

In accordance with another aspect of the present invention, there isprovided a method for preparing a multilayer of nanocrystals-polymercomprising the steps of: (i) coating nanocrystals surface-coordinated bya photosensitive compound, or a mixed solution of a photosensitivecompound and nanocrystals surface-coordinated by a material misciblewith the photosensitive compound, on a substrate, drying the coatedsubstrate, and exposing the dried substrate to UV light to form a firstmonolayer of nanocrystals; (ii) forming a first polymer monolayer on thefirst monolayer of nanocrystals; and (iii) repeating the procedure ofstep (i) and/or step (ii) to form one or more monolayers of nanocrystalsand/or one or more polymer monolayers on the first polymer monolayer.

In accordance with yet another aspect of the present invention, there isprovided an organic-inorganic hybrid electroluminescence device using amultilayer of nanocrystals or a multilayer of nanocrystals-polymerprepared by any one methods of the present invention, as a luminescentlayer disposed between a pair of electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view showing the structure of a multilayer ofnanocrystals prepared by a method of the present invention;

FIG. 2 is a schematic view showing the structure of an organic-inorganichybrid electroluminescence device using a multilayer of nanocrystalsprepared in accordance with one embodiment of the present invention, asa luminescent layer;

FIG. 3 is a schematic view showing the structure of an organic-inorganichybrid electroluminescence device using a multilayer ofnanocrystals-polymer prepared in accordance with another embodiment ofthe present invention, as a luminescent layer;

FIG. 4 shows photoluminescence spectra of a monolayer of nanocrystalsand multilayers of nanocrystals, prepared in Example 1 of the presentinvention, according to increasing number (3 and 5) of the monolayers;

FIG. 5 shows electroluminescence spectra of an organic-inorganic hybridelectroluminescence device using a multilayer of nanocrystals-polymer asa luminescent layer, fabricated in Example 2 of the present invention,and an organic-inorganic hybrid electroluminescence device using amonolayer of nanocrystals as a luminescent layer; and

FIG. 6 is a graph showing the luminescent efficiency of anorganic-inorganic hybrid electroluminescence device using a multilayerof nanocrystals-polymer as a luminescent layer, fabricated in Example 2of the present invention, and an organic-inorganic hybridelectroluminescence device using a monolayer of nanocrystals as aluminescent layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in moredetail with reference to the accompanying drawings.

Preparation of Multilayer of Nanocrystals

First, nanocrystals surface-coordinated by a photosensitive compound, ora mixed solution of a photosensitive compound and nanocrystalssurface-coordinated by a material miscible with the photosensitivecompound is coated on a substrate, dried, and then exposed to UV lightto form a first monolayer of nanocrystals. Thereafter, the aboveprocedure is repeated to form one or more monolayers of nanocrystals onthe first monolayer of nanocrystals, thereby preparing a multilayer ofnanocrystals. In the method of the present invention, the nanocrystalsare prepared by a wet chemistry technique, and the surface of thenanocrystals is coordinated by an appropriate dispersant in order tominimize agglomeration between the nanocrystals and preventprecipitation of the agglomerated nanocrystals.

In this manner, when the nanocrystals surface-coordinated by thephotosensitive compound are coated on a substrate and exposed to ahigh-energy light source, such as UV light, a crosslinking reactiontakes place between photosensitive functional groups of thephotosensitive compound. This crosslinking reaction enables theformation of a network structure of the nanocrystals, resulting in theformation of a monolayer of nanocrystals insoluble in solvents.

On the other hand, when a uniformly mixed solution of a photosensitivecompound and nanocrystals surface-coordinated by a dispersant containingno photosensitive functional group but miscible with the photosensitivecompound is coated on a substrate and exposed to a high-energy lightsource, such as UV light, a crosslinking reaction takes place betweenphotosensitive functional groups of the photosensitive compound. Thiscrosslinking reaction enables the inclusion of the nanocrystals in anetwork structure, resulting in the formation of a first monolayer ofnanocrystals insoluble in solvents. Thereafter, the above procedure isrepeated to form one or more monolayers of nanocrystals on the firstmonolayer, thereby preparing a multilayer of nanocrystals. The structureof the multilayer of nanocrystals is schematically shown in FIG. 1.

Nanocrystals usable in the present invention may include allnanocrystals prepared from metal precursors by a wet chemistrytechnique. For example, the nanocrystals may be prepared by adding acorresponding metal precursor to an organic solvent in the absence orpresence of a dispersant, and growing crystals at a predeterminedtemperature.

That is, nanocrystals usable in the present invention include most ofthe nanocrystals prepared by a wet chemistry technique, such as metalnanocrystals and semiconductor nanocrystals. As suitable nanocrystals,there may be mentioned, for example: metal nanocrystals, such as Au, Ag,Pt, Pd, Co, Cu, Fe, Al, Ni, Ir, Rh, Ru, and Mo; Group II-VI compoundsemiconductor nanocrystals, such as CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe,HgS, HgSe and HgTe; and Group III-V compound semiconductor nanocrystals,such as GaN, GaP, GaAs, InP, InAs, InN, AlN, AlP, and AlAs; and GroupIV-VI compound semiconductor PbS, PbSe, PbTe. As needed, there can beused a mixture of two or more nanocrystals, for example: nanoparticlesin a simple mixture state of at least two nanocrystal compounds; fusedcrystals in which at least two compound crystals are partially presentin the same crystal structure, e.g., core-shell structured crystals andgradient-structured crystals, and alloys of at least two nanocrystalcompounds.

According to the method of the present invention, the monolayersconstituting the multilayer of nanocrystals may be composed of one kindof nanocrystals, or two or more kinds of nanocrystals. That is, themultilayer of nanocrystals may have the same or different layerconstitution.

The photosensitive compound coordinated to the surface of thenanocrystals is a compound wherein a photoreactive functional group(e.g., carbon-carbon double bond or acryl group) is selectively bondedto a linker (e.g., cyanide, thiol (SH), amino, carboxylic acid group orphosphonic acid group) through an alkylene, amide, phenylene,biphenylene, ester or ether group.

The nanocrystals surface-coordinated by the photosensitive compound canbe prepared by obtaining nanocrystals from a corresponding metalprecursor, dispersing the obtained nanocrystals in an organic solvent,and treating the dispersion with the photosensitive compound. Thetreatment with the photosensitive compound is not especially limited,but is preferably carried out by refluxing the dispersion of thenanocrystals in the presence of the photosensitive compound. The refluxconditions, including time and temperature, and the concentration of thephotosensitive compound can be properly controlled according to the kindof the photosensitive compound coordinated to the surface of thenanocrystals, the dispersing solvent and the nanocrystals. For instance,nanocrystals are surface-coordinated by a dispersant (e.g.,mercaptopropanol) having a reactive end group, and are then reacted witha photosensitive compound (e.g., methacryloyl chloride) capable ofreacting with the reactive end group of the dispersant, therebypreparing nanocrystals surface-coordinated by the photosensitivecompound.

Alternatively, nanocrystals directly surface-coordinated by aphotosensitive compound may be prepared by adding a corresponding metalprecursor to an organic solvent in the presence of the compound having aphotosensitive functional group, and growing crystals at a predeterminedtemperature. The kind of the organic solvent, the crystal-growthtemperature and the concentration of the precursor can be appropriatelyvaried according to the kind of the photosensitive compound, and thekind, size and shape of the desired nanocrystals.

The coating of the nanocrystals on a substrate may be carried out by aspin coating, dip coating, spray coating or blade coating process, butis not especially limited thereto.

Considering the melting point of the solvent dispersing thenanocrystals, the drying of the coated substrate may be carried out at20-300° C. and preferably 40-120° C. for complete removal of thesolvent.

The exposure of the dried substrate may be carried out by a contactexposure or non-contact exposure process. In addition, the energy forphotosensitization treatment is dependent on the thickness of themonolayer, and is preferably in the range of 50-850 mJ/cm². When theexposure energy is out of this range, a crosslinking reaction is notlikely to take place, or there is a risk of damage to the monolayer.Considering absorption wavelength of the photosensitive group andcrosslinking reaction conditions, light sources usable for the lightexposure preferably have an energy in the range of 100-800 W at aneffective wavelength of 200-500 nm and preferably 300-400 nm.

Preparation of Multilayer of Nanocrystals-Polymer

The present invention is also directed to a method for preparing amultilayer of nanocrystals-polymer. First, nanocrystalssurface-coordinated by a photosensitive compound, or a mixed solution ofa photosensitive compound and nanocrystals surface-coordinated by amaterial miscible with the photosensitive compound is coated on asubstrate, dried, and exposed to UV light to form a first monolayer ofnanocrystals (step (i)). Next, a first polymer monolayer is formed onthe first monolayer of nanocrystals (step (ii)). Thereafter, step (i)and/or step (ii) are repeated to form one or more monolayers ofnanocrystals and/or one or more polymer monolayers on the first polymermonolayer, thereby preparing a multilayer of nanocrystals-polymer.

Like the method for preparing a multilayer of nanocrystals, a firstmonolayer of nanocrystals is formed in step (i). Thereafter, a polymermonolayer is formed on the monolayer of nanocrystals. At this time,since the exposed monolayer of nanocrystals is highly stable insolvents, a monolayer composed of a polymer or a polymer precursor canbe formed on the first monolayer of nanocrystals. Using a solventcausing no damage to the monolayer composed of a polymer or a polymerprecursor, another monolayer of nanocrystals is formed on the monolayercomposed of a polymer or a polymer precursor to prepare the finalmultilayer of nanocrystals-polymer.

Consequently, the multilayer of nanocrystals-polymer can be prepared bycoating nanocrystals on a substrate, exposing the coated substrate to UVlight to form a first monolayer of nanocrystals insoluble in solvents,forming a polymer monolayer on the first monolayer of nanocrystals,forming another monolayer of nanocrystals on the polymer thin film, andrepeating the overall procedure. At this time, at least one kind ofnanocrystal and at least one kind of polymer may be used. In addition,the multilayer of nanocrystals-polymer can be prepared by alternatelylayering the monolayer of nanocrystals and the polymer monolayer, oralternately layering several monolayers of nanocrystals and severalpolymer monolayers.

Fabrication of Organic-Inorganic Hybrid Electroluminescence DeviceComprising Multilayer of Nanocrystals or Multilayer ofNanocrystals-Polymer

The multilayer of nanocrystals or the multilayer of nanocrystals-polymerprepared by the method of the present invention is used as a luminescentlayer of an organic-inorganic hybrid electroluminescence device.

An organic-inorganic hybrid electroluminescence device according to oneembodiment of the present invention is schematically shown in FIG. 2. Asshown in FIG. 2, the multilayer of nanocrystals is used as a luminescentlayer disposed between a pair of electrodes. Specifically, theorganic-inorganic hybrid electroluminescence device comprises asubstrate, a hole injection electrode, a hole transport layer, aluminescent layer (the multilayer of nanocrystals), an electrontransport layer, and an electron injection electrode layered in thisorder from the bottom.

An organic-inorganic hybrid electroluminescence device according to oneembodiment of the present invention is schematically shown in FIG. 3. Asshown in FIG. 3, the multilayer of nanocrystals-polymer is used as aluminescent layer disposed between a pair of electrodes. Specifically,the organic-inorganic hybrid electroluminescence device comprises asubstrate, a hole injection electrode, a hole transport layer, aluminescent layer (the multilayer of nanocrystals-polymer), an electrontransport layer, and an electron injection electrode layered in thisorder from the bottom.

The organic-inorganic hybrid electroluminescence device of the presentinvention using the multilayer of nanocrystals or the multilayer ofnanocrystals-polymer as a luminescent layer may further comprise anelectron blocking layer and/or a hole blocking layer interposed betweenthe luminescent layer and the hole transport layer or between theluminescent layer and the electron transport layer.

The organic-inorganic hybrid electroluminescence devices shown in FIGS.2 and 3 are fabricated by a method comprising the steps of: (i) forminga hole injection electrode (anode) on a substrate; (ii) forming a holetransport layer (HTL) on the hole injection electrode; (iii) forming themultilayer of nanocrystals or the monolayer of nanocrystals-polymerprepared by the method of the present invention (i.e. a luminescentlayer), on the hole transport layer; (iv) forming an electron transportlayer (ETL) on the multilayer; and (v) forming an electron injectionelectrode (cathode) on the electron transport layer.

Examples of suitable substrates of the organic-inorganic hybridelectroluminescence device according to the present invention includethose commonly used in the art of organic electroluminescence devices. Aglass or transparent plastic substrate is preferably used in terms ofsuperior transparency, superior surface smoothness, easy handlingproperties, and excellent waterproofness. Specific examples arepreferably glass, polyethyleneterephthalate, and polycarbonatesubstrates. The thickness of the substrate is preferably in the range of0.3-1.1 mm.

The hole injection electrode can be formed of a conductive metal or anoxide thereof, for example, indium tin oxide (ITO), indium zinc oxide(IZO), nickel (Ni), platinum (Pt), gold (Au), silver (Ag), or iridium(Ir). The substrate on which the electrode is formed is commonly washedwith solvents, such as a neutral detergent, deionized water, acetone andisopropyl alcohol, and is then subjected to UV-ozone and plasmatreatment.

As materials for the hole transport layer, polymeric materials causingno damage to the multilayer of nanocrystals or the multilayer ofnanocrystals-polymer are preferably used. As the polymeric materials,there may be mentioned, for example, poly(3,4-ethylenedioxythiophene)(PEDOT)/polystyrene para-sulfonate (PSS), polytriphenylamine,polyphenylenevinylene and derivatives thereof, and polyfluorene andderivatives thereof. The thickness of the hole transport layer ispreferably in the range of 10 to 100 nm.

The multilayer of nanocrystals or the multilayer of nanocrystals-polymeris prepared by the method of the present invention.

In order to prepare the multilayer of nanocrystals-polymer used as aluminescent layer of the organic-inorganic hybrid electroluminescencedevice according to the present invention, the polymer is preferablyselected from materials capable of transporting holes and electrons orcontrolling the transportation rate. Examples of suitable polymersinclude poly(3,4-ethylenedioxythiophene) (PEDOT)/polystyrenepara-sulfonate (PSS), polytriphenylamine, polyphenylenevinylene andderivatives thereof, polyfluorene, polyaniline, and polypyrrole andderivatives thereof. The formation of at least one polymer monolayercapable of transporting holes and electrons or controlling thetransportation rate between monolayers of nanocrystals enables an activecontrol in transportation of holes and electrons, allowing themonolayers of nanocrystals to emit light or enhancing the luminescentefficiency of the device.

Low- and high-molecular weight materials commonly used in the art can beused to form the electron transport layer, unlike the materials for thehole transport layer. As materials constituting the electron transportlayer, there may be mentioned, for example, oxazoles, isooxazoles,triazoles, isothiazoles, oxydiazoles, thiadiazoles, perylenes, andaluminum complexes, including tris(8-hydroxyquinoline)-aluminum (Alq3),bis(2-methyl-8-quinolinolato) (p-phenyl-phenolato) aluminum (Balq) andbis(2-methyl-8-quinolinolato)(triphenylsiloxy) aluminum (III) (Salq).The thickness of the electron transport layer is preferably between 10nm and 100 nm.

Examples of suitable materials for the electron blocking layer and/orthe hole blocking layer include those commonly used in the art. Specificexamples are 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ),2,9-dimethyl-1,10-phenanthroline (BCP), phenanthrolines, imidazoles,triazoles, oxadiazoles, and aluminum complexes. The thickness of theelectron blocking layer and the hole blocking layer is preferably in therange of 5 nm to 50 nm.

As a material for the electron injection electrode, there can be used ametal having a low work function, such as I, Ca, Ba, Ca/Al, LiF/Ca,LiF/Al, BaF₂/Al, BaF₂/Ca/Al, Al, Mg, or a Ag—Mg alloy. The thickness ofthe electron injection electrode is preferably in the range of 50 nm to300 nm.

The fabrication of the organic-inorganic hybrid electroluminescencedevice of the present invention does not require particular fabricationapparatuses and methods, except the formation of the luminescent layer.The organic-inorganic hybrid electroluminescence device of the presentinvention can be fabricated in accordance with conventional fabricationmethods using common luminescent materials.

Hereinafter, the present invention will be specifically explained withreference to the following examples. However, these examples are givenfor the purpose of illustration and are not to be construed as limitingthe scope of the invention.

PREPARATIVE EXAMPLE 1 Preparation of CdSeS NanocrystalsSurface-Coordinated by Photosensitive Compound Containing Double Bond

16 g of trioctylamine, 0.5 g of oleic acid and 0.4 mmol of cadmium oxidewere charged simultaneously into a 125 ml flask equipped with a refluxcondenser. The temperature of the mixture was raised to 30° C. withstirring. Separately, a selenium (Se) powder was dissolved in trioctylphosphine (hereinafter, referred to as TOP) to obtain an Se-TOP complexsolution (Se concentration: about 0.25M), and a sulfur (S) powder wasdissolved in TOP to obtain an S-TOP complex solution (S concentration:about 1.0M). 0.9 ml of the S-TOP complex solution and 0.1 ml of theSe-TOP complex solution were rapidly fed to the previous mixture, andthen reacted for 4 minutes with stirring. After the reaction wascompleted, the reaction mixture was cooled to room temperature asrapidly as possible. Ethanol as a non-solvent was added to the reactionmixture, and the resulting mixture was then centrifuged. After theobtained precipitates were separated from the mixture by decanting thesupernatant, 1 wt % of the precipitates were dispersed in toluene toprepare a dispersion of CdSeS nanocrystals. A luminescence peak in thephotoluminescence spectrum of the nanocrystals was observed around 550nm, and the nanocrystals emitted yellowish green light under a UV lampat 365 nm.

EXAMPLE 1 Preparation of Multilayer of CdSeS NanocrystalsSurface-Coordinated by Photosensitive Compound

After the toluene dispersion of CdSeS nanocrystals (1 wt %) prepared inPreparative Example 1 was dropped onto a glass substrate cleaned withisopropyl alcohol (IPA), spin coating was performed at 2,000 rpm for 30seconds. The coated glass substrate was heated on a heating plate at 50°C. to form a monolayer of the nanocrystals. The monolayer was placed ina UV exposure system at an effective wavelength of 200-300 nm, and thenUV light (800 W) was irradiated to the monolayer for about 300 seconds.The toluene dispersion of nanocrystals was dropped onto the UV-exposedmonolayer, which was then spin-coated, dried and exposed to UV lightunder the same conditions as the previous formation procedure of themonolayer of nanocrystals. Thereafter, this procedure was repeated fourtimes to prepare a multilayer (5-layer) of nanocrystals. FIG. 4 showsphotoluminescence spectra of the multilayers of nanocrystals accordingto increasing number of the monolayers. As shown in FIG. 4, oneluminescence peak in all the photoluminescence spectra was observedaround 550 nm, and the luminescence intensity increased with increasingnumber of the monolayers. Further, the luminescence peak had a fullwidth at half maximum (“FWHM”) of approximately 36 nm.

EXAMPLE 2 Fabrication of Electroluminescence Device ComprisingMultilayer of Nanocrystals-Polymer as Luminescent Layer

An ITO-patterned glass substrate was sequentially washed with a neutraldetergent, deionized water, acetone and isopropyl alcohol, and was thensubjected to UV-ozone treatment.Poly(9,9′-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB) wasspin-coated on the substrate to form a hole transport layer having athickness of 50 nm, and then baked. The toluene solution of CdSeSnanocrystals (1 wt %) prepared in Preparative Example 1 was spin-coatedon the hole transport layer and dried to form a monolayer having athickness of about 5 nm. After the monolayer was placed in a UV exposuresystem at an effective wavelength of 200-300 nm, UV (800 W) light wasirradiated thereto for about 200 seconds. A solution of TFB wasspin-coated on the exposed monolayer, and baked to obtain a polymermonolayer. The CdSeS nanocrystals were spin-coated on the polymermonolayer to prepare a multilayer of nanocrytals-polymer as aluminescent layer.(3-4-Biphenylyl)4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) wasdeposited on the luminescent layer to form a hole blocking layer, andthen tris(8-hydroxyquinoline)-aluminum (Alq3) was deposited thereon toform an electron transport layer having a thickness of about 30 nm. LiFand aluminum were sequentially deposited on the electron transport layerto thicknesses of 1 nm and 200 nm, respectively, to fabricate anorganic-inorganic hybrid electroluminescence device.

One luminescence peak in the electroluminescence spectrum of theorganic-inorganic hybrid electroluminescence device was observed around550 nm, and the FWHM was approximately 90 nm. In addition, the intensityof the peak was 73 Cd/m² and the luminescent efficiency was 0.32 Cd/A.

FIG. 5 shows electroluminescence spectra of the organic-inorganic hybridelectroluminescence device using the multilayer of nanocrystals-polymer(TFB)-nanocrystals as a luminescent layer, fabricated in Example 2, andan organic-inorganic hybrid electroluminescence device using a monolayerof nanocrystals as a luminescent layer. FIG. 6 is a graph showing theluminescent efficiency of the organic-inorganic hybridelectroluminescence device using the multilayer of nanocrystals-polymer(TFB)-nanocrystals as a luminescent layer, fabricated in Example 2, andan organic-inorganic hybrid electroluminescence device using a monolayerof nanocrystals as a luminescent layer.

As shown in FIG. 5, the electroluminescence spectrum of theorganic-inorganic hybrid electroluminescence device using the multilayerof nanocrystals-polymer-nanocrystals as a luminescent layer is similarto that of the organic-inorganic hybrid electroluminescence device usinga monolayer of nanocrystals as a luminescent layer. However, it can beconfirmed from FIG. 6 that the luminescent efficiency of theorganic-inorganic hybrid electroluminescence device using the multilayerof nanocrystals-polymer-nanocrystals as a luminescent layer is 5.5 timesthat of the organic-inorganic hybrid electroluminescence device using amonolayer of nanocrystals as a luminescent layer.

As apparent from the foregoing, the use of the multilayer ofnanocrystals or the multilayer of nanocrystals-polymer as a luminescentlayer enables fabrication of an organic-inorganic hybridelectroluminescence device having enhanced luminescent efficiency andluminescence intensity. Particularly, the electrical properties of theelectroluminescence device using the multilayer of nanocrystals-polymeras a luminescent layer can be controlled by the properties of theselected polymer.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for preparing a layer of nanocrystals comprising; preparingthe nanocrystals coordinated with the photosensitive compound or a mixedsolution of the nanocrystals and the photosensitive compound; coatingthe nanocrystals coordinated with the photosensitive compound on asubstrate or coating the mixed solution of the nanocrystals and thephotosensitive compound on a substrate, and then drying the coated film;and then exposing the dried film to UV light to form a solvent resistantcross-network of the nanocrystal layer, wherein the photosensitivecompound is a compound with a photoreactive functional group selectivelybonded to a linker through an alkylene amide, phenylene, biphenylene,ester, or ether group.
 2. The method according to claim 1, wherein thenanocrystals are at least one kind of nanocrystals selected from thegroup consisting of metal nanocrystals, Group II-VI compoundsemiconductor nanocrystals, Group III-V compound semiconductornanocrystals, and Group IV-VI compound semiconductor nanocrystals, or amixture of two or more nanocrystals selected from the group consistingof metal nanocrystals, Group II-VI compound semiconductor nanocrystals,Group III-V compound semiconductor nanocrystals, and Group IV-VIcompound semiconductor nanocrystals, wherein a mixture is in a form of asimple mixture, fused crystals or an alloy.
 3. The method according toclaim 2, wherein the metal nanocrystals are selected from the groupconsisting of Au, Ag, Pt, Pd, Co, Cu, Fe, Al, Ni, Ir, Rh, Ru and Mo. 4.The method according to claim 2, wherein the Group II-VI compoundsemiconductor nanocrystals are selected from the group consisting ofCdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe and HgTe, wherein the GroupIII-V compound semiconductor nanocrystals are selected from the groupconsisting of GaN, GaP, GaAs, InN, InP, InAs, AlN, AlP, and AlAs, andwherein the Group IV-VI compound semiconductor nanocrystals are selectedfrom the group consisting of PbS, PbSe, and PbTe.
 5. The methodaccording to claim 1, wherein the functional group is a carbon-carbondouble bond or an acryl group.
 6. The method according to claim 1,wherein the linker is a cyanide group, thiol group, amino group,carboxylic acid group, or phosphonic acid group.
 7. The method accordingto claim 1, wherein drying is carried out at 20˜300° C.
 8. The methodaccording to claim 1, wherein light exposure is carried out using alight source having an energy of 100˜800 W at a wavelength of 200˜500nm.
 9. The method according to claim 1, wherein energy forphotosensitization treatment upon the light exposure is in the range of50˜850 mJ/cm².
 10. The method according to claim 1, further comprisingdisposing a monolayer of polymer on the layer of nanocrystals.
 11. Amethod for preparing a layer of nanocrystals-polymer comprising:preparing the nanocrystals coordinated with the photosensitive compoundor a mixed solution of the nanocrystals and the photosensitive compound;coating the nanocrystals coordinated with the photosensitive compound ona substrate or coating a mixed solution of the nanocrystals and thephotosensitive compound on a substrate, and then drying the coated film;and then exposing the dried film to UV light to form a solvent resistantcross-network of the nanocrystal layer, wherein the photosensitivecompound is a compound with a photoreactive functional group selectivelybonded to a linker through an alkylene amide, phenylene, biphenylene,ester, or ether group. forming a solvent resistant polymer monolayer onthe nanocrystal layer.
 12. The method according to claim 11, wherein thenanocrystals are at least one kind of nanocrystals selected from thegroup consisting of metal nanocrystals, Group II-VI compoundsemiconductor nanocrystals, Group III-V compound semiconductornanocrystals, and Group IV-VI compound semiconductor nanocrystals, or amixture of two or more nanocrystals selected from the group consistingof metal nanocrystals, Group II-VI compound semiconductor nanocrystals,Group III-V compound semiconductor nanocrystals, and Group IV-VIcompound semiconductor nanocrystals, wherein a mixture is in a form of asimple mixture, fused crystals or an alloy.
 13. The method according toclaim 12, wherein the metal nanocrystals are selected from the groupconsisting of Au, Ag, Pt, Pd, Co, Cu, Fe, Al, Ni, Ir, Rh, Ru and Mo. 14.The method according to claim 12, wherein the Group II-VI compoundsemiconductor nanocrystals are selected from the group consisting ofCdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe and HgTe, wherein the GroupIII-V compound semiconductor nanocrystals are selected from the groupconsisting of GaN, GaP, GaAs, InN, InP, InAs, AlN, AlP, and AlAs, andwherein the Group IV-VI compound semiconductor nanocrystals are selectedfrom the group consisting of PbS, PbSe, and PbTe.
 15. The methodaccording to claim 11, wherein the functional group is a carbon-carbondouble bond or an acryl group.
 16. The method according to claim 11,wherein the linker is a cyanide group, thiol group, amino group,carboxylic acid group, or phosphonic acid group.
 17. The methodaccording to claim 11, wherein drying is carried out at 20˜300° C. 18.The method according to claim 11, wherein light exposure is carried outusing a light source having an energy of 100˜800 W at a wavelength of200˜500 nm.
 19. The method according to claim 11, wherein energy forphotosensitization treatment upon the light exposure is in the range of50˜850 mJ/cm².